Streptococcosis: causes, symptoms and treatment

Streptococcosis (Streptococcus iniae / S. agalactiae) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: very high.

Overview

Gram-positive coccobacillus causing meningoencephalitis, popeye, and erratic swimming. ZOONOTIC — bacteremia and cellulitis reported in humans handling infected fish. Causative agent: Streptococcus iniae / S. agalactiae. Transmission: water. Incubation: 3-14 days. Reported mortality without treatment: very high.

Symptoms

spinning or whirling swimming
popeye (often unilateral)
darkening
skin hemorrhages
lethargy
sudden death

Causes

Outbreaks are typically triggered by chronic stress, poor water quality, temperature swings, overcrowding, or the introduction of unquarantined fish. The pathogen spreads via free-swimming or waterborne stages in shared water.

Diagnosis

Diagnosis is based on clinical signs (lesions, hemorrhages, behaviour) combined with bacterial culture and Gram-staining where available. Differentiate from co-infections with other Gram-negative pathogens; antibiotic sensitivity testing improves treatment success against Streptococcus iniae / S. agalactiae.

Treatment

Effective treatment requires isolating affected fish in a quarantine tank, identifying the pathogen, administering the appropriate active substance at the correct dose and duration, and supporting recovery with stable water parameters and nutrition.

Step 1: Quarantine

Set up a bare-bottom quarantine tank with a mature sponge filter, heater, and aeration. Match temperature and pH to the display tank, and acclimate fish slowly. A bare bottom simplifies daily siphoning and prevents medication from being absorbed by substrate.

Step 2: Medication

Erythromycin or amoxicillin in feed. Erythromycin 50 mg/kg or amoxicillin 80 mg/kg in feed for 10-14 days; Strep is Gram-positive so aminoglycosides are weak. (duration: 10-14 days)

Step 3: Recovery

After medication, perform a 30-50% water change and run fresh activated carbon for 24-48 hours to remove residues. Continue feeding a high-quality, varied diet with vitamins and immunostimulants. Reintroduce fish to the display tank only after at least one week without recurrence of symptoms.

Prevention

wear gloves when handling sick fish
avoid temperature stress >28 C in tilapia
quarantine and treat new stock
avoid wild-caught feeders